1. Field of the Invention
The present invention relates to a method of manufacturing an optical fiber preform having at least a core portion, and to an optical fiber produced by drawing the optical fiber preform.
2. Description of the Background Art
Generally, an optical fiber is manufactured by drawing an optical fiber preform. The optical fiber preform is produced by an outside vapor deposition method (OVD method), modified chemical vapor deposition method (MCVD method), vapor-phase axial deposition method (VAD method), or rod-in-collapse method, etc. Of various kinds of optical fibers, the tolerance of profile shape parameters required to achieve desired characteristics is narrow in optical fibers such as a dispersion-shifted fiber, dispersion flattened fiber, dispersion compensating fiber, and highly nonlinear dispersion-shifted fiber (including highly nonlinear dispersion flattened fiber), etc. It is difficult to produce these optical fibers in a manner such that the profile shape parameters are in a desirable range throughout over a long length and the optical characteristics are desirable throughout over a long length.
Japanese Patent Application Publication No. 2002-293563, Japanese Patent Application Publication No. 2003-40636, and Japanese Patent Application Publication No. 2003-20239 respectively disclose techniques with which an attempt is made to solve the above-mentioned problems by grinding the outer periphery of, or by providing a cladding material to, an optical fiber preform or an intermediate preform (semi-manufactured product at an intermediate stage of manufacturing an optical fiber preform). However, there are cases in which these techniques are insufficient for solving the problems, since in the case of the above-mentioned optical fibers, an error in the measured values of the core portion diameter and the refractive index occasionally increases. For example, in the above-mentioned optical fibers, the diameter of a core portion is as small as 10 μm or less, and also the diameter of a region which is to become the core portion is often small at the stage immediately before fiber drawing of an optical fiber preform, whereby correct measurement of shape of the core portion is difficult. Likewise, the relative refractive index difference Δ of the core portion is often as high as 1% or more relative to the cladding portion, and it is difficult to measure a refractive index profile correctly. Furthermore, there are cases in which the outer diameter of a preform which is determined based on the result of such measurement significantly differs from the true target outer diameter, thereby causing a wide variation in the optical characteristics of an optical fiber produced from the preform by fiber-drawing.
Various applications such as wavelength conversion, optical amplification, pulse compression, an optical switch, generation of supercontinuum light (white light), a multi-wavelength light source, chirp compensation of a light source, etc. have been investigated and developed by using the generation of optical nonlinear effects in a highly nonlinear dispersion-shifted fiber, such as four-wave mixing (FWM), self-phase modulation (SPM), cross-phase modulation (XPM), and modulation instability. Particularly, in a case where wavelength conversion or optical parametric amplification (OPA) is implemented, the variation of chromatic dispersion and the variation of zero dispersion wavelength are matters of significant concern (See, for example, A. Legrand, et al., Technical Digest OAA2003, WD2, p. 261; or A. Mussot, et al., ECOC2004 Proceedings Vol. 2, Paper Tu3.3.7, p. 190).
The invention disclosed in Japanese Patent Application Publication No. 2000-347228 aims to suppress the fluctuation of chromatic dispersion by making the length of an optical fiber to be shorter. However, the efficiency in the generation of nonlinearity is better in an optical fiber having a longer length to some extent. In Japanese Patent Application Publication No. 2004-29441, it is claimed that the variation in the chromatic dispersion is preferably 3 ps/nm/km or less. Even in such case, when the dispersion slope is assumed to be +0.02 ps/nm2/km or so, the variation in the zero dispersion wavelength reaches 150 nm. In S. Watanabe, et al., ECOC'98, PD-Paper p. 85, the broad band wavelength conversion is achieved using a polarization-maintaining highly nonlinear dispersion shifted fiber in which the variation of zero dispersion wavelength is within ±0.5 nm over a fiber length of 1000 m. It is expected to increase the bandwidth by further decreasing the variation.